This invention relates to the scan line bow in an optical scanning system and, more particularly, to a heating element for adjusting the curvature of a cylindrical mirror, typically the wobble correction mirror, to compensate and correct the scan line bow in an optical scanner or to approximately equalize the scan line bows in a multiple beam optical scanning system.
A raster output scanner (or ROS) conventionally has a multi-faceted polygon mirror that is rotated about its central axis while an intensity-modulated beam is directed to the rotating polygon at a predetermined angle. The light beam is reflected by a facet and thereafter focussed to a xe2x80x9cspotxe2x80x9d on a photosensitive recording medium. The rotation of the polygon causes the spot to scan linearly across the photosensitive medium in a scan direction. Meanwhile, the photosensitive medium is advanced relatively more slowly than the rate of the scan in a slow scan direction which is orthogonal to the scan direction. In this way, the beam scans the photosensitive medium in a raster scanning pattern.
Specifically, reference is made to FIG. 1, wherein there is disclosed a conventional prior art raster scanning system 10. A laser diode light source 12 emits a modulated coherent light beam 14 which is collimated by a multi-element optical collimator 16. The resulting collimated beam 14 passes through a cross-scan cylindrical lens 18. The cylindrical lens 18 focuses the light beam 14 in the sagittal or cross scan plane onto a facet 20 of the rotating multi-faceted polygon mirror 22 while maintaining the collimation of the scan portion of the beam. The light beam 14 thus forms a line on the facet 20.
The light beam 14 is reflected from the facet 20 and the reflected light beam is still collimated in the scan plane and is now diverging in the cross-scan plane. The reflected beam 14 then passes through an f-theta scan lens 24 consisting of a negative plano-spherical lens 26, a positive plano-spherical lens 28 and a cross-scan cylindrical lens 30. This f-theta scan lens configuration converges the beam 14 in the scan axis.
After passing through the f-theta scan lens 24, the light beam 14 is then reflected off a cylindrical wobble correction mirror 32. The mirror 32 is positive and cylindrical in the cross-scan plane and flat in the scan plane. Thus, the wobble mirror converges the previously diverging cross-scan portion of the light beam 14 but allows the converging cross-scan portion of the light beam 14 focused by the f-theta lens 24 to pass through unaffected. The reflected beam 14 is focussed onto a scan line 34 on a photosensitive medium 36 such as a photoreceptor.
A raster scanner typically includes a number of lenses and mirrors to accommodate a specific design. Unavoidable imprecision in the shape and/or mounting of these optical elements will inevitably introduce certain anomalies in the quality of the scan line on the photoreceptor. Also, different errors can be introduced to the light beam by different optical elements of the raster scanner in which case the error is cumulative. One such anomaly is bow.
Bow is an undesirable character of a scan line when the light beam is offset in the cross-scan direction from the ideal horizontal straight line in the scan direction of the scan line on the recording medium. The scan line bow occurs because the magnification of the optical system of the ROS varies across the cross-scan direction as the beam propagates through the optical system.
An example of a bow is shown by scan line 50 in FIG. 2. Depending on the types of imprecision in the ROS, the two end points of the bowed scan line will bend in an opposite direction relative to the mid-point of the bowed scan line with the scan line deviating relative to a straight, bow free line 52. A bow 50 (FIG. 2) with the end points of the scan line situated downward is called a frown and a bow 54 (FIG. 3) with the end points of the scan line situated upward is called a smile.
The scan lines from multiple raster scanners are registered or positioned in parallel lines on the photosensitive medium. Another undesirable character with multi-beam scanners is called differential bow. Differential bow happens in multi-beam raster scanners in which the multiple light beams are nominally off-axis relative to each other and relative to the straight bow free scan line.
For example, referring to FIG. 4, if the bow scan line 56 of one light beam from one ROS happens to be a smile while the bow scan line 58 of the other beam from another ROS happens to be a frown, then the separation 60 between the two beams varies across the scan. This phenomenon is called a differential bow. Differential bow can have different shapes such as two scan lines 62 and 64 in FIG. 5 with the same shape bow but a different amount of bow on each scan line. It is desirable to have all the scan lines straight and parallel so that the beam separation across the scan line would be uniform and therefore, the differential bow would be eliminated.
One solution to the problem of differential bow in a multiple beam ROS is found in U.S. Pat. No. 5,543,829 to Fisli, commonly assigned as the present application and herein incorporated by reference. The Fisli patent applies physical pressure directly to the back and front surfaces of the wobble correction mirror to adjust the curvature of the mirror horizontally. The resulting curved mirror will compensate for scan line bow.
This prior art solution to scan line bow requires physically bending the wobble correction mirror and maintaining the wobble correction mirror in a bent position. The mechanics for the device to physically bend the wobble correction mirror add considerable weight, alignment difficulty and complexity to the ROS.
Differential bow arises from multiple beams from a single ROS. Scan line bow is a different problem. Scan line bow arises in both single and multiple beam ROS""s and is the overall curvature of the scan line relative to the ideal of a straight scan line.
It is an object of the present invention to provide an apparatus for thermally adjusting the curvature of a cylindrical mirror to compensate and correct the scan line bow in a single beam optical scanning system.
It is another object of the present invention to provide an apparatus for thermally adjusting the curvature of a cylindrical mirror to approximately equalize the scan line bows in a multiple beam multiple optical scanning system.
According to the present invention, a heating element adjusts the curvature of a cylindrical wobble correction mirror to compensate and correct the scan line bow for an optical scanner. A current applied through the heating element along one side of the mirror will cause the mirror to bend vertically for a horizontal beam to adjust the scan line bow for a single beam and approximately equalize the scan line bow for multiple beams. A rheostat will vary the current to vary the applied heat to control the amount of bend in the mirror.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.